It might be less why garou didnt die from the punch and more why he didn't die from the power leaving his body, and why the power left in the first place.
as you said, god is outside causalty. When the timetravel happened he was aware of it and was already manipulating things in the background for void to be in the right time and the right place. We know its him guiding void, because if it was purely void's concious decision, then void would have appeared in the original timeline when god blessed garou introduced himself.
But no, it just so happened that he didn't appear. Blast was already there at the point the timeline diverged, and unless void only appears because his ability to sense god's powers allows him to differentiate when its inside someone and when its leaving, then he should atleast check what was going on and at that point his presence noticed by blast.
In the new timeline however, he is INSTANTLY there, PERFECTLY timed.
The last two times god's power left someone, they either died from being burned from the inside or it transmutes them to salt. The varying effects might have to do with what the target could endur so its adjusted accordingly, but what I believe happened is that GOD instead of wasting time bothering to use the powers inside garou to kill him, he instead put all his effort in making it leave garou as fast as possible. Blast is there after all and every moment its trying to do anything else on garou's body is a moment its not outside for void to suck